To increase the basic understanding of the cardiac function to improve techniques to aid in the diagnosis of heart disease, we propose to conduct research on an improved method of evaluating myocardial function in active patients. This approach involves the use of a vary compact, vestlike instrument which uses nuclear medicine techniques to measure the ejection fraction of the high ventricle on a beat by beat basis. The system is designed to be worn by ambulatory patients over an 8 to 16 hour period and uses and on-board cassette recorder to store both the nuclear and the corresponding ECG data. The basic concept, explored in collaboration with Mass. General Hospital, appears quite promising. However the present nuclear sensor is too large and is relatively insensitive to be suitable for clinical work. We propose to develop a new alternative sensor which will be sufficiently sensitive, reliable, and compact to make this concept a reality. In Phase I we will demonstrate the possibility of making such sensors and in Phase II complete the sensor and associated microelectronics for a working clinical prototype.